Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide in the presence of heterogeneous or homogenous catalysts into equimolar amounts of phenol and acetone, a co-product. However, the world demand for phenol is growing more rapidly than that for the acetone co-product. In addition, due to developing shortages in supply, the cost of propylene is likely to increase.
Thus, a process that avoids or reduces the use propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol. For example, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam, and nylon 6.
It is known that phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is via benzene hydroalkylation in which benzene is contacted with hydrogen in the presence of a catalyst such that part of the benzene is converted into cyclohexene which then reacts with the remaining benzene to produce the desired cyclohexylbenzene. One such method is disclosed in U.S. Pat. No. 6,037,513, in which the catalyst comprises a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product in roughly equimolar amounts.
There are, however, a number of problems associated with producing phenol via cyclohexylbenzene rather than the cumene-based Hock process. One such problem is that, even with the most selective catalysts, the process converts only a portion of the benzene feed per pass and the reaction product inherently contains a non-negligible amount of cyclohexane, which is normally converted into benzene at a downstream step. Thus, a viable commercial benzene hydroalkylation plant will likely include recycle of both unreacted benzene from the primary hydroalkylation step but also of benzene produced in downstream processing steps. This leads to an additional problem in that both sources of benzene tend to contain impurities that can adversely affect the hydroalkylation catalyst. For example, deleterious impurities in fresh benzene include water, light aliphatics, olefins, diolefins, styrene, toluene and other aromatics, oxygenated organic compounds, sulfur-containing compounds, nitrogen-containing compounds, and oligomeric compounds. Impurities in the recycle benzene that may affect the hydroalkylation catalyst include toluene, biphenyl, and light aliphatics/olefins/diolefins.
International patent application publication No. WO 2011/146167 A2 provides a general description of the influence of impurities in the benzene and, more particularly, the hydrogen employed in benzene hydroalkylation processes. In the case of benzene, the relevant impurities are said to include nitrogen and sulfur compounds and are removed by treatment with adsorbents, such as molecular sieves.
However, in view of the wide range of potential impurities encountered in a commercial setting, there remains a need for an improved process for removing the impurities from the fresh and recycle benzene feed steams to benzene hydroalkylation units and the present invention seeks to address this need.